Oncogenic viruses are important agents in the etiology of cancer and other diseases. The etiologic agent of adult T-cell leukemia (ATL) is human T- lymphotropic virus type 1 (HTLV-I). The HTLV-I attributable risk for ATL was particularly elevated for younger cases, highlighting the importance of early age at infection. Infective dermatitis of children is an HTLV-I associated syndrome. American Indian populations in North, Central, and South America are a natural reservoir for HTLV-II infection. Large cell granulocytic leukemia may be an HTLV-II associated malignancy. Studies of human immunodeficiency virus type 1 (HIV-1) have documented a long latency between infection and disease with a growing spectrum of associated malignancies. Non-Hodgkin's lymphoma (NHL) is an emerging and important outcome of HIV-1 induced immunodeficiency accounting for approximately 10% of current NHL in the U.S. The risk of NHL in a large cohort of HIV- infected hemophiliacs was increased 200-fold but there was no excess of non-AIDS cancers. In the AIDS cancer match registry, among 14,000 cancer cases the majority were Kaposi's sarcoma and NHL, but other candidate malignancies are being evaluated. Hepatitis C virus is linked to hepatocellular carcinoma and may interact with HIV-1 to accelerate hepatic dysfunction in hemophiliacs. Herpes- viruses are cofactors in several malignancies, including those which are HIV-1 associated. Epstein-Barr virus (EBV) was identified in the Reed-Sternberg cells of tumors obtained from patients with Hodgkin's disease. Chronic fatigue syndrome (CFS), a disease reportedly linked to cancer, was shown to be a heterogeneous group of illnesses, rendering its association with cancer questionable.